Atherosclerosis is one of the chief causes of morbidity and mortality in the United States and many other countries of the world. (Zuckerbraun et al., Arch Surg. 137:854–861 [2002]; Kibbe et al., Circ Res. 86:829–33 [2000]). This process can result in limiting the flow of blood to the heart, kidneys and the peripheral vessels, to name a few. Current approaches to the treatment of lesions in the arteries include coronary artery bypass graft (CABG) surgery and angioplasty with or without the placement of a stent. The latter may serve as a vehicle for drug delivery, as is currently being tested in clinical trials. A number of pharmacological agents that affect platelet function or provide anticoagulant properties have so far failed to reduce re-occlusion or intimal hyperplasia. (Kibbe et al., Circ Res. 86:829–33 [2000]).
Cardiovascular diseases, however, are the result of complex pathophysiologic processes that involve the expression of many proteins and molecules that can adversely affect the grafted vessel (Shears et al., J. Am Coll Surg., 187(3):295–306 [1998]; Ross et al., Nature, 362:801–9 [1993]). Approximately 15–30% of patients receiving vein grafts for coronary or peripheral vascular disease require follow-up treatment, either in the form of angioplasty or new grafts.
Thrombomodulin (TM) is an integral membrane glycoprotein expressed on the surface of endothelial cells (Sadler et al., Trhomb Haemost., 78:392–95 [1997]). It is a high affinity thrombin receptor that converts thrombin into a protein C activator. Activated protein C then functions as an anticoagulant by inactivating two regulatory proteins of the clotting system, namely factors Va and VI[I]a (Esmon et al., Faseb J., 9:946–55 [1995]). The latter two proteins are essential for the function of two of the coagulation proteases, namely factors Ixa and Xa. TM thus plays an active role in blood clot formation in vivo and can function as a direct or indirect anticoagulant.
There are several other proteins or enzymes that have shown to reduce the process of intimal hyperplasia, whose evolution is the cause of late graft failure. For instance, Nitric oxide synthase, an enzyme expressed by endothelial cells has been shown in animal models to inhibit intimal hyperplasia, especially the inducible enzyme (iNOS) (Salmaa et al., Lancet, 353:1729–34 [1999]; Palmer et al., Nature, 327:524–26 [1987]; Kubes et al., PNAS USA., 88:4651–5 [1991]).
Animal studies shown that cytoxic gene transfection utilizing the Herpes Simplex Virus thymydine kinase gene delivered via an adenoviral vector was able to inhibit intimal hyperplasia (Steg et al., Circulation, 96:408–11 [1997]).
Vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF) and platelet derived growth factor (PDGF) have all been shown to promote reendothelization and enhance the healing of vascular injury and help limit intimal hyperplasia. (Ban Bellle et al., Biochem Biophs Res Commun., 235:311–16 [1997]; Salyapongse et al., Tissue Engineering 26(4):663–76 [1999]).
A gene therapy approach is currently under clinical investigation. It involves the injection, directly into heart muscles, of an adenoviral vector delivery system containing the gene for the expression of vascular endothelial growth factor (VEGF). This is being tested in patients whose coronary vessels are not amenable to standard grafting procedures. However, some recent adverse clinical events demonstrated that injection of large quantities of adenovirus vectors is associated with significant risks. Accordingly, there still exist a need for a method to effectively introduce therapeutic genes, such as TM, into vascular tissues.